Problem: $\dfrac{9}{8} - \dfrac{9}{6} = {?}$
$ = {\dfrac{9 \times 3}{8 \times 3}} - {\dfrac{9 \times 4}{6 \times 4}} $ $ = {\dfrac{27}{24}} - {\dfrac{36}{24}} $ $ = \dfrac{{27} - {36}}{24} $ $ = -\dfrac{9}{24}$